Why doesn't this fail?

[Alan Kennedy]

Hi all,

I'm writing some code that uses threads to iterate over a
read-only object that is shared between all threads.

The object contains lists of items. The object also has a
method, which iterates over the lists, using a local variable
(not an object data member).

I would expect that the local
variable would be shared between threads. So, without mutual
exclusion, I expect that using that local variable should cause
errors in a multi-threaded situation.

But it doesn't. Here is some sample code that illustrates what
I mean.

==========
import threading
import time

limit = 100000

class datastruct:

  def __init__(self):
    self.ls = []
    for ix in xrange(limit):
      self.ls.append(ix)

  def visit(self, markerdict):
    for x in self.ls:
      del markerdict[x]

sharedstruct = datastruct()

class thr(threading.Thread):

  def __init__(self):
    threading.Thread.__init__(self)

  def run(self):
    self.markerdict = {}
    for ix in xrange(limit):
      self.markerdict[ix] = 1
    sharedstruct.visit(self.markerdict)
    print "Thread %s missed %s" % (self.getName(),
self.markerdict.keys())

if __name__ == "__main__":
  for x in range(10):
    t = thr()
    t.start()
=======

Each thread keeps a thread-private list of values that have been
processed, so that it can know at the end what it missed out on.

When I run this code, each of the threads visits every value, with no
problem! Why?

When I change the datastruct.visit method to use an object data member
as the loop variable, like so

def visit(self, markerdict):
  for self.x in self.ls:
    del markerdict[self.x]

lots of KeyErrors are generated, as I would expect, because the value
"self.x" is definitely shared between all threads.

So, despite my expectations, it appears that local variables are indeed
per-thread?

I wanted to check, before declaring my code thread-safe. I feel uneasy
"just hoping".

Regards,

alan kennedy
----------------------------------------------
check http headers here: xhaus.com/headers
email alan:              xhaus.com/mailto/alan